Among the III-nitride semiconductors, single crystalline AlN is a material that features a direct bandgap of about 6 eV. In addition, as AlN has a larger bandgap than other nitrides such as GaN and InN, it is possible to engineer the bandgap energy through alloying of AlN with Ga or In. As a consequence, III-nitride semiconductors enable short wavelength light emission in the ultraviolet (UV) spectral range and are expected to be utilized for the fabrication of white light LEDs, UV-LEDs for sterilization applications, lasers for high-density optical disc storage applications, and light emitting sources for laser communications. To form semiconductor devices such as light emitting devices, it is necessary to form a multilayer structure including active layers between an n-type semiconductor layer electrically connected to an n-electrode and a p-type semiconductor layer electrically connected to a p-electrode. It is important for all layers to have high crystallinity, i.e., to have few dislocations and point defects that could adversely affect light emission efficiency.
Sapphire substrates are often used for III-nitride based LEDs from the viewpoint of stable supply, cost, and UV transparency. It is possible to obtain III-nitride semiconductor devices by using highly transparent sapphire as a substrate. However, due to the fact that there is a difference in lattice constant between the III-nitride LED device layers and the sapphire substrate, a large density of dislocations of about 109 cm−2 is generated at the interface between the substrate and the device structure, and it is a known problem in the art that this elevated dislocation density adversely affects the light emission efficiency and the lifetime of LED devices. Therefore, it is desirable to use AlN or GaN single crystals as substrates for III-nitride based LEDs, since the use of these native III-nitride substrates minimizes the difference in lattice constant between substrate and device layers. In addition, since dislocations present in the substrate tend to propagate into the device layers, the use of III-nitride substrates with low dislocation density is desirable. Furthermore, as AlN and GaN substrates possess high thermal conductivity, they help dissipate the Joule heat generated by the process of current injection in the light emitting layers. It is understood that heat dissipation enhances device lifetime. In particular, the use of AlN as a substrate has been reduced to practice.
The following production method is known for AlN single crystals with low dislocation density, which are desirable for the aforementioned applications. First, an AlN substrate is prepared from an AlN single crystal grown using a sublimation method, and then serves as a seed crystal substrate. Subsequently, a single crystalline AlN layer is deposited on the AlN substrate by hydride vapor phase epitaxy (HVPE). Thereafter, the HYPE-grown single crystalline AlN layer is separated from the original substrate, and the HVPE-grown single crystalline AlN layer then serves as a substrate for the fabrication of light emitting devices (see Japanese Pat. Appl. Pub. No. 2006-16294).
The main incentive to form a semiconductor layer (e.g., an AlN single crystalline layer) on a sublimation-grown AlN substrate by HVPE consists of the fact that the impurity levels can be easily controlled during HVPE growth. For example, when an AlN layer is grown by HYPE, it is relatively easy to reduce the level of impurities originating from structural elements of the reactor, since the crystal growth temperature in an HYPE reactor is much lower than in a sublimation growth reactor. As a result of lower contamination levels, it is possible to enhance the optical transparency of single crystalline AlN layers. In addition, when the single crystalline AlN layer is intentionally doped with dopants such as silicon in order to impart conductivity, the desired dopant concentration can easily be controlled by supplying a source of dopants along with the precursors for AlN.
When compared to generally similar vapor phase epitaxy methods, HVPE offers additional advantages. For example, when compared with a vapor phase epitaxy process such as MOCVD (metalorganic chemical vapor deposition), the HVPE method offers higher growth rates of single crystalline AlN layers since higher concentrations of precursors for AlN can be supplied.
Therefore, by using the HVPE method, it is possible to deposit single crystalline AlN layers with desirable optical properties and electrical characteristics at relatively high productivity levels. However, according to the studies performed by the inventors, when producing semiconductor devices (LEDs) according to Japanese Pat. Appl. Pub. No. 2006-16294, it was found that there was room for improvement. In particular, when LEDs were fabricated on the single crystalline AlN layers prepared according to the HYPE growth procedure described in Japanese Pat. Appl. Pub. No. 2006-16294, it was found that only part of the LEDs exhibited acceptable emission characteristics, while other LEDs suffered from low optical output or device failure.
In addition to the production method discussed thus far, a modified HVPE method for the production of single crystalline AlN layers of higher crystallinity was devised. Specifically, combining reactor heating from the outside with additional, local heating of the seed crystal substrate enables higher growth temperatures, which, in turn, enable the formation of highly crystalline AlN layers (see Japanese Pat. Appl. Pub. Nos. 2005-343705 and 2008-19130). However, LEDs fabricated on single crystalline AlN layers obtained by this improved method still showed some performance issues in terms of low optical output and device failure.
In comparison with the HVPE method described above, the sublimation method is known to obtain highly crystalline and highly transparent AlN (see, for example, WO 2010/001803). According to this method, it is possible to produce high-quality AlN with low dislocation density and residual impurity level. However, AlN obtained by this method showed a high refractive index, probably due to the production method, and thus is too highly reflective for use as a substrate for LEDs. As a consequence, there remains room for improvement in terms of light extraction efficiency. In addition, single crystalline AlN layers grown by this method showed as a particular characteristic that the transparency becomes particularly poor as the refractive index decreases.
Although the crystal described by Roskovcová et al. (Physica Status Solidi (b), 20, K29 (1967)) has lower refractive index than the AlN single crystal described in WO 2010/001803, there is a strong absorption band between the band edge of AIN and a wavelength of about 300 nm. Therefore, this AlN single crystal cannot be used as a substrate for UV-LEDs. Similarly, crystals with lower refractive index than the AlN single crystal described in WO 2010/001803 are reported by Jiang et al. (Optical Materials, 32, 891 (2010)). The AlN single crystals described by Jiang et al. are films grown on the surface of sapphire substrates, and it is, therefore, straightforward to predict that a large number of dislocations will have formed due to the lattice mismatch between the sapphire substrate and the AlN single crystal. Furthermore, it appears that there is strong absorption in the deep UV region, and thus this material cannot be used as a substrate for deep-UV LEDs.
Accordingly, there remains a need in the art for highly transparent single crystalline AlN layers that contribute to the improvement of optical light output and to the reduction of device failure of light emitting devices, meaning the highly transparent single crystalline AlN layers are useful as device substrates for LEDs.